1. Field of the Invention
The present invention relates to apparatus and methods for making thermoplastic tubular articles, which are open at both ends, which are dimensionally stable up to a specific elevated temperature and which are made from crystallizable polymers, and to the articles made thereby, particularly tubular bodies of saturated linear polyester materials such as polyethylene terephthalate, which are intended for processable food and beverage containers.
2. Description of the Prior Art
While biaxial drawing of a thermoplastic saturated linear polyester material, such as polyethylene terephthalate, improves its mechanical properties by inducing a degree of biaxial orientation, the material will, however, shrink if heated above the temperature at which it was drawn. It is known that this tendency of biaxially drawn polyester sheet material to shrink can be reduced by heat-setting the material. Methods and apparatus for forming biaxially oriented tubular articles, which are heat-shrink resistant and which may therefore be used as bodies for processable food containers, are disclosed in our co-pending UK Patent No. 8037137 (Publication No. 2089276A) and UK Patent Application No. 8310966. The second of these applications is specifically concerned with heat-treating an open-ended body, but the apparatus suffers the disadvantage of thermal cycling of certain components, with the result that energy is wasted.
European Patent Application No. 82630109.5 (Publication No. 0081451) filed by the Goodyear Tire and Rubber Company is concerned with a process and apparatus where the formation, by plug-forming, and heat-treatment of PET tube are combined. In contrast, in the present invention the heat-treatment is separate from the forming step, with the result that open-ended tubular articles can be produced.